The electrotechnical industry predominantly employs acid anhydride or amine cross-linked epoxy resins without solvents as mechanically-thermally high-quality reaction resin molded materials. However, the thermal and mechanical properties required of those reaction resin molded materials has become more and more stringent because the temperature range in which the molded materials are used has been expanded more and more toward high, as well as, low stress temperatures. It is difficult to increase the dimensional heat resistance of the molded materials and maintain high mechanical strength and temperature cycle resistance with epoxy resins cross-linked with acid anhydrides. Increasing the dimensional heat resistance of molded materials can be achieved by increasing the cross-link density, which increases the glass transition temperature. However, this approach significantly degrades the mechanical properties of the molded materials.
It is known that cross-linking EP/IC resins results in OX/ICR molded materials having a high dimensional heat resistance (DE-AS 1 115 922: Col. 5, lines 9 to 14, and DE-AS 1 963 900: Col. 1, lines 4 to 13 and 48 to 60).
U.S. Pat. No. 4,070,416 describes processes for manufacturing OX/ICR molded materials involving EP/IC resins which have a formula mole ratio EP:IC smaller than 1; these EP/IC resins are gelled by means of suitable reaction accelerators, i.e., hardening catalysts, at temperatures of 70.degree. to 130.degree. C. and then post-hardened at temperatures up to more than 200.degree. C. It is also pointed out there that, for the manufacture of OX/ICR molded materials with high glass transition temperatures and at the same time good mechanical properties, the most advantageous formula mole ratio EP:IC is in the range of 0.2 to 0.7.
Investigations of our own, while confirming the high dimensional heat resistance of OX/ICR molded materials produced by the above-mentioned process, clearly show that the mechanical properties of these molded materials are not adequate for many applications, for instance, for the manufacture of insulations with larger insulation wall thicknesses or for the encapsulation of electrical or electronic components which are subjected to high temperature cycle stresses. If, for instance, EP/IC resins (bisphenol-A-diglycidyl ether, diphenylmethane diisocyanate and 66% quartz powder) with a formula mole ratio EP:IC between 0.2 to 0.7 are cross-linked with tertiary amines or imidazoles as reaction accelerators, according to the cross-linking conditions given, extremely brittle OX/ICR molded materials are obtained, the mechanical properties of which get increasingly worse with increasing concentration of diphenylmethane diisocyanate in the EP/IC resin.
It is therefore understandable that it has already been proposed to improve the mechanical properties of OX/ICR molded materials by incorporating flexibilizing or elastifying components. U.S. Pat. No. 3,979,365 suggests prepolymeric oxazolidinones with terminal epoxy groups as polyepoxy components for modifying EP/IC resins. These oxazolidinone prepolymers are prepared by reaction of polyisocyanates with an excess of a polyepoxy component. However, the viscosity of the prepolymers is so high that they are difficult to use without solvents. U.S. Pat. No. 4,100,118 suggests using polyglycidyl esters of dimerized fatty acids mixed with polyepoxides in EP/IC resins. While these polyglycidyl esters have low viscosity, they exhibit a poor mixing behavior in the EP/IC resins. It has also been proposed, to add copolymers of butadiene and acrylonitrile to the EP/IC resins as elastifying agents in U.S. Pat. Nos. 4,128,531 and 4,130,546. These copolymers have functional groups such as hydroxyl and carboxyl groups, whereby the carboxyl groups can be reacted with polyepoxides. Due to their high viscosity, these copolymers, especially in the case of filler-containing EP/IC resins, are hardly usable and are difficult to process because they exhibit a strong tendency toward segregating from the EP/IC resin.
It is an object of the invention to provide a method which, permits the simple manufacture of heat-resistant OX/ICR molded material, i.e., reaction resin molded materials containing oxazolidinone and isocyanurate rings starting with reaction resin mixtures of polyepoxides and polyisocyanates, i.e. EP/IC resins, to produce molded materials with good mechanical properties. The method does not require flexibilizing or elastifying additives and provides readily processable reaction resin mixtures.